1. Field of the Invention
The present invention relates to an antenna device in which a ground pattern and a pattern antenna are parallel arranged on a printed substrate, and more particularly to an antenna device in which a pattern antenna is arranged in the vicinity of a short side portion of a ground pattern of an approximately rectangular shape.
2. Description of the Related Art
In recent years, short distance wireless technologies of Bluetooth (trademark) and the like have been commercialized to perform wireless data communication between a plurality of electronic devices at home and the like. For example, when an image captured by a digital camera is downloaded to a personal computer (so-called PC) or printed by a printer if these short distance wireless technologies are adopted, the digital camera does not need to be connected to an opposite electronic device through a cable since desired image data can be transmitted from an antenna device embedded in the digital camera to an opposite PC or printer. For example, in a state in which a portable music player is put in a satchel, a bag, or the like when outside if these short distance wireless technologies are adopted, a remote operation of the player may be performed through an antenna device embedded in a terminal controller of a headphone and simultaneously a music reproduced by the player can be listened to through the headphone.
As the antenna device to be used in this short distance wireless communication as described above, a structure capable of being cost-effectively produced in a compact size is preferable. As an antenna device capable of satisfying these requirements, there is a well-known structure in which a ground pattern and a pattern antenna of a predetermined shape are conventionally formed by a metal foil or the like on a printed substrate. The pattern antenna is arranged adjacent to one side end portion of the ground pattern, and operates as, for example, an inverse F type antenna when a power feed signal is supplied to a feed point (for example, see JP-A-2004-343285 (Pages 3 and 4, FIG. 1)).
However, in the conventional antenna device in which the ground pattern and the pattern antenna are parallel arranged on the printed substrate as described above, a radiation element configures the pattern antenna, arranged in the vicinity of the one side end portion of the ground pattern, to which the power feed signal is supplied. However, when the pattern antenna is adjacent to the short side portion of the approximately rectangular shape of the ground pattern, an antenna gain is lowered at both end sides in a longitudinal direction of the ground pattern. This is because a high-frequency current easily flows into the ground pattern in the longitudinal direction when the pattern antenna is powered on and excited. If a casing for containing the printed substrate has an elongate shape, the ground pattern is mostly formed in an approximately rectangular shape.
However, the antenna device in which the casing has the elongate shape and the gain is low at both the end sides in the longitudinal direction of the ground pattern is not necessarily convenient for use. That is, when users holding the casing of the elongate shape perform short distance wireless communication (data communication) with an opposite electronic device, many users cause one end side in the longitudinal direction of the casing to be toward the opposite device. Accordingly, when the antenna gain is low in this direction, communication error may easily occur. For example, in the case of a conventional general antenna device in which a ground pattern 2 of a smaller rectangular shape and an inverse F type pattern antenna 3 adjacent to a short side portion of the ground pattern 2 are arranged on a printed substrate 1 of a rectangular shape and electric waves are radiated from the pattern antenna 3 by supplying a power feed signal to a power feed portion 3a as shown in FIG. 9, directional characteristics within a flat surface along the printed substrate 1 are formed as shown in FIG. 10. In FIG. 10, an azimuth angle of 0 degree is present in a side of the ground pattern 2 as viewed from the pattern antenna 3. That is, it can be seen that a straight-line direction connecting azimuth angles of 0 and −180 degrees corresponds to the longitudinal direction of the ground pattern 2 and a sufficient gain is not obtained in this direction.
As is apparent from a dip of the directional characteristics shown in FIG. 10, a gain of the antenna device shown in FIG. 9 is extremely lowered in a specific direction (at the azimuth angle of 120 or −90 degrees). Accordingly, there is a problem in that it is difficult to practically realize an omnidirectional antenna device available in any direction. For this reason, for example, there is a worry that a music being listened to is interrupted in mid course when the antenna device is in a specific direction in a headphone or the like in which the music is listened to through the antenna device.